In recent years, III/V nitride materials, mainly GaN, InGaN, and AlGaN, have received much attention as semiconductor materials. Due to their continuous and variable direct band gap from 1.9 to 6.2 eV, excellent physical and chemical stability, and high saturation electron mobility, the III/V nitride materials are the most preferred materials for optoelectronic devices such as laser devices and light-emitting diodes.
Due to the limitation in the growth technologies of GaN, however, large area of GaN materials are mostly grown on sapphire substrates. Although the GaN grown on a sapphire substrate has high quality and wide applications, the development of GaN based semiconductor devices is largely limited by the non electro-conductivity and poor thermal-conductivity of the sapphires. In order to avoid such disadvantages, methods have been invented to replace the sapphires substrate after the growth of GaN based devices on sapphires. The GaN film can be transferred to a better heatsink bonding or be used as a kind of homo-epitaxial substrate after the substrate is lifted off. A commonly applied method for the removal of sapphire is laser lift-off technology.
The laser lift-off (LLO) technique was first implemented by HP company on AlGaInP/GaAs LED because the GaAs substrate leads to a very high light absorption loss inside the LED. The light-emitting efficiency can be increased nearly 2 times by removing the GaAs substrate and then adhered to a transparent GaP substrate. GaN based laser lift-off technology was first introduced by M. K. Kelly et al. in US in 1996 as part of hetero epitaxial growth technologies. A thick GaN film grown by hydrogen vapor epitaxial (HVPE) is lifted off using a third-harmonic frequency YAG laser from the sapphire substrate. In 1998, W. S. Wong et al. made GaN based LED and laser diodes using LLO technology. The laser lift-off techniques have received extensive attention.
The laser lift-off technology resolves a series of problems associated with GaN based LED on sapphire substrate, including: heat emission, current crowding, and low light-emitting efficiency. LLO is the most prospective technology to overcome the above mentioned obstacles in lighting applications. Firstly, by transferring the epitaxial wafer to a heat sink having high thermal-conductivity, the heat dissipation of the LED device is much improved and the LED junction temperature is lowered. The reduction of LED junction temperature significantly improves light-emitting efficiency and stability of the LED, and increases LED's lifetime. The lift-off technology can also substantively reduce fabrication costs in conventional processes such as etching, wafer abrasion, wafer scribing, etc. The cost is also reduced by repeated use of the lift-off substrate.
The IX-1000 laser lift-off apparatus of US JPSA company is the main commercialized laser lift-off apparatus at the present time. It applies high-power KrF quasi-molecule laser with 248 nm wavelength and 25-38 ns pulse width. Through accurate control on the energy and uniformization of the light beam energy distribution, the laser focuses on the GaN buffer layer to decompose the GaN into gallium and nitrogen so as to implement the separation of GaN from the substrate. A YAG third harmonic frequency solid-state laser with Q switch is also used besides the KrF quasi-molecule laser, mainly by the M. K. Kelly group in the US and R. H. Horng group in Taiwan. The solid-state laser can achieve a rather high pulse energy through Q switch technology. Moreover, it is convenient to be maintained. However, such techniques have not been commercialized due to their technical limitations.
The above-mentioned lift-off methods have the following characteristics:
1. Applying lift-off techniques chip by chip. Substrate separation is achieved by a large laser spot (with laser spot size larger than or equal to the chip size).
2. The size of the laser spot is dependent on the size of the device (chip).
3. The laser spot has a uniform energy distribution with a flat range at the top of the energy distribution.
4. The laser spot has high energy density of generally more than 0.6 J/cm2.
5. The positional registration between each device unit and the laser spot is accomplished by movable platform and visual recognition system.
Although the above two techniques have resolved certain issues in lift-off technologies after industrial applications in the last several years, at the same time, several problems, as described below, still remain:
1. The KrF laser cannot maintain energy stability of the laser pulses. The energy fluctuations of the laser pulses can damage the structures within the device and lower yields.
2. The parameters of the laser lift-off cannot be adjusted accurately because the size of the laser spot must be adjusted according to chip size. As a result, lift off cannot be conducted consistently.
3. In recent years, the industry always questioned such large area lift-off method because of the large laser spot. The GaN decomposes simultaneously in the irradiation area which leads to large stress and deformation in the decomposition area, so as to imperil the quality and the service life of the chip. Although manual adjustment enables the GaN decomposition as accurate as possible, such macroscopic adjustment is difficult to meet the microcosmic requirement. In the meantime, it makes the laser adjustment more difficult because of the discreteness of the pulse energy of the KrF quasi-molecule laser.